1. Field of the Invention
The present invention relates to frequency shifting techniques suitable for a demodulator of a receiver in a multi-carrier communications system.
2. Description of the Related Art
In next generation mobile communications systems, much attention is focused on IMT-2000 (International Mobile Telecommunications-2000) system defined by ITU-R TG 8/1. There have been proposed several systems such as W-CDMA (Wideband-Code Division Multiple Access) and cdma2000, which may employ a multi-carrier scheme to allow high-speed data transmission.
The cdma2000 system is designed to realize upward compatibility with cdmaOne conforming to IS-95 and is likely to employ a multi-carrier scheme in downlink transmission. An example of the multi-carrier scheme in CDMA communications is shown in FIG. 10. In this example, it is assumed that the frequency offset of sub-carriers (Carrier−1 and Carrier+1) from the center carrier is 1.25 MHz and the chip rate is 1.2288 Mcps. In the cdmaOne communications, data can be transmitted at 14.4 kbps using a single carrier (center carrier). In contrast, the multi-carrier cdma system allows a maximum data rate of 43.2 kbps using three carriers.
To successfully receive such a multi-carrier signal, the simplest is to provide a different receiver dedicated to each of a plurality of carriers. However, the number of receivers to be needed increases as the number of carriers increases, resulting in increased amount of hardware and increased power consumption.
Another solution is that a single receiver is provided to receive signals on all the carriers and a digital baseband processor individually processes the received signals according to carrier frequencies. There have been proposed several methods for handling a received signal for each of a plurality of carriers.
In Japanese Patent Unexamined Publication No. 7-221806, a demodulator employing time division multiplexing scheme has been disclosed. More specifically, the respective carriers are identified by time slots of the time division multiplexing scheme and I- and Q-component signals for each carrier are frequency-shifted to produce baseband I- and Q-component signals for the center carrier by phase rotation computation.
In Japanese Patent Unexamined Publication No. 8-46654, a demodulator employing a carrier selection means at an input stage has been disclosed. More specifically, one of a plurality of carriers is selected according to carrier designation data. Only a signal of the selected carrier is subjected to quadrature frequency conversion to produce I- and Q-component signals and the I- and Q-component signals for each carrier are frequency-shifted to produce baseband I- and Q-component signals for the center carrier by phase rotation computation.
In Japanese Patent Unexamined Publication No. 10-79718, a demodulator employing a Fast Fourier Transformer (FFT) for used in an OFDM (orthogonal frequency division multiplex) receiver has been disclosed. The FFT can be used to separate a plurality of carriers.
In the above prior arts, however, a ROM and a complex multiplier are needed for phase shifting and therefore there is disclosed no effective means for avoiding complication of the circuit, increase of the circuit scale, and increase of power consumption. A multi-carrier receiver having a small circuit scale and saving power is not realized.